1. Field of the Invention
The present invention relates to an equalization method suitable for use in a digital cellular telecommunications system.
2. Description of the Prior Art
There are known different digital cellular telecommunications systems in which a central switching office (fixed office) and mobile units (automobile telephones) are interconnected by radio waves. One of such digital cellular telecommunications systems is a time-division multiple access system. According to the time-division multiple access system, each channel of 900 MHz bandwidth is allotted six time slots, for example, for signal reception, and in each time slot a signal is received for 20 msec. within a time period of 120 msec. Similarly, six time slots for signal transmission are assigned to each channel, and a signal is transmitted in each time slot. The reference reception carrier frequency and the reference transmission carrier frequency are different from each other.
When a received signal (burst signal) is equalized by an equalizer in the central switching office or the mobile units, since the signal is received in a very short period of time (20 msec. in the above example), a signal portion corresponding to an initial portion of the received signal tends to drop from the equalized output signal from the equalizer. Furthermore, because the mobile unit moves at high speed, the characteristics of the received signal are likely to change abruptly due to Rayleigh fading or frequency-selective fading. Therefore, demodulating such received signal requires an automatic equalizer of quick convergence.
One known algorithm having high-speed convergence characteristics is the RLS (Kalman) algorithm. Since, however, this algorithm requires a vast amount of calculation O(u.sup.2) (u: the number of taps) and high calculation accuracy, it cannot easily be implemented by a digital signal processor (DSP).
Another known algorithm which requires less calculation accuracy and a smaller amount of calculation is the LMS algorithm. However, the LMS algorithm is not suitable for digital mobile telecommunication systems as its convergence speed is low.
The applicant has proposed, in Japanese patent application No. 2-130752, an equalization method (automatic equalization method) for supplying an input signal (burst signal) to a series circuit of plural delay means, multiplying the input signal and delayed output signals from the delay means by coefficients, and adding the products into an equalized output signal. The proposed equalization method comprises the steps of supplying the input signal to the series circuit, transmitting the input signal in a normal direction entirely through the series circuit to delay the input signal successively with the delay means, thereafter transmitting the input signal in an opposite direction through a predetermined number of delay means to delay the input signal successively with the predetermined number of delay means, thereafter transmitting the input signal in the opposite direction entirely through the series circuit to delay the input signal successively with the delay means, thereafter transmitting the input signal in the normal direction entirely through the series circuit to delay the input signal successively with the delay means, detecting an amplitude error of the equalized output signal each time the input signal is transmitted in the series circuit in one of the normal direction, the opposite direction, and the normal direction, and calculating coefficients by which to multiply the input signal and the delayed output signals from the respective delay means, depending on the detected amplitude error, in order to minimize the amplitude error.
However, if the transmission characteristics of the input signal (burst signal) vary, then the proposed equalization method tends to fail to converge, but become divergent.